1) Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus that includes a mechanism for separating transfer sheets.
2) Description of the Related Art
One of the image forming methods adopted in image forming apparatuses, such as the copier, printer, facsimile apparatus, or printing press involves the steps of using a developer to convert a latent image formed on a photosensitive drum, which is used as a latent image carrier, to a visible image, and transferring the visible image to a transfer sheet by means of electrostatic image transfer.
Using a toner to adhere electrostatically to the electrostatic latent image on the photosensitive drum is a well-known conversion process of latent image to visible image. The toner image obtained as a result of this method is transferred by means of electrostatic image transfer to the transfer sheet such as a recording paper. Fixing converts the toner image to a reproduction.
A transfer apparatus is used in the step involving the transfer of the toner image on the photosensitive drum to the transfer sheet. The transfer apparatus transfers the toner image to the transfer sheet by conveying the transfer sheet in such a way that the transfer sheet adheres against the toner image on the photosensitive drum, and impressing a bias.
Transfer apparatuses having a conveying belt for carrying the transfer sheet, and a pair of rollers supporting the conveying belt that impress a transfer bias on the belt that is of the opposite polarity to the toner are well known.
Once the toner image is transferred, the transfer sheet separates from the photosensitive drum and is conveyed towards a fixing apparatus. The separation of the transfer sheet from the photosensitive drum depends on the transfer sheet's flexural rigidity and the so-called nerve of the transfer sheet. In other words, the transfer sheet adhering to the photosensitive drum separates from the photosensitive drum due to a curvature separation resulting from the direction of movement of the transfer sheet at the transfer position, the direction of movement of the transfer sheet when adhering to the photosensitive drum, the fact that the direction of the transfer sheet when adhering to the photosensitive drum corresponds to the curvature of the photosensitive drum, and the transfer sheet's own form restorative force. However, when the force of the electrostatic adhesion of the transfer sheet to the photosensitive drum exceeds the transfer sheet's nerve, the transfer sheet fails to separate from the photosensitive drum.
To counter the problem, structures have been proposed wherein a separating pawl having a pointed end is provided on the surface of the photosensitive drum in a position beyond where toner image transfer takes place, or a structure is provided wherein uniform exposure is carried out prior to image transfer to reduce the background potential of the photosensitive drum while at the same time neutralizing the transfer sheet in the transfer apparatus. Such a technology is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2002-268498 (Paragraph 0036).
However, in spite of reducing the adhesive force of the transfer sheet towards the photosensitive drum by reducing the potential of the photosensitive drum by means of pre-transfer exposure, the ability of the transfer sheet to separate from the photosensitive drum may be adversely affected by the setting of the transfer bias of the transfer apparatus.
The transfer bias has charge attributes that are of opposite polarity to that of the toner adhering to the photosensitive drum and may, for instance, be impressed from the underside of the conveying belt that carries the transfer sheet. However, if too much charge is injected into the conveying belt, the transfer sheet lying on the surface of the conveying belt may get charged, and its polarity is reversed. Same polarity in the transfer sheet and the photosensitive drum causes the transfer sheet and the photosensitive drum to repel each other and discourages electrostatic adhesion.
Often, the transfer bias is constantly impressed in order to maintain sufficient quantity of charge on the transfer sheet side with the aim of enhancing the efficiency of transfer. Consequently, the charge on the conveying belt side also tends to increase, resulting in the reversal of polarity of the charges on the transfer sheet side.
The transfer sheet thus repelled from the photosensitive drum may adhere to the photosensitive drum due to a residual charge on the surface of the photosensitive drum. The passage of the transfer sheet against the photosensitive drum at this stage may cause the separating pawl to scum the transfer sheet.
The scum of the transfer sheet is caused by the separating pawl attached to photosensitive drum picking up some of the toner adhering to the photosensitive drum and the toner being transferred back to the leading edge of the transfer sheet from the separating pawl.
Scum is not limited alone to the leading edge of the transfer sheet. When the entire surface of the transfer sheet adheres to the photosensitive drum and is conveyed while in contact with the separating pawl, not only does an unwanted stripe appears on the portion of the transfer sheet that is caught on the separating pawl, the unfixed image transferred to the transfer sheet is also faint.
The effect of residual charge on the surface of the photosensitive drum is explained next. Until the time neutralization is completed in the cleaning stage, the photosensitive drum retains a certain amount of charge in spite of reduction in the quantity of charge due to the effect of transfer bias. The retention of charge on the photosensitive drum causes the toner to adhere to the photosensitive drum, leading to scum (surface staining) of the photosensitive drum.
The scum of the photosensitive drum is scraped off and the photosensitive drum is neutralized in the cleaning stage so that a clean photosensitive drum is available for the next round of image formation. However, if the scum exceeds a certain amount, that is, if the charge amount between the toner and the residual charge is balanced according to the adhesion of toner, which determines the concentration of scum, the residual charge is unlikely to act on the transfer sheet and cause deterioration in its separability.
However, in the case where only charge remains and the amount of scum is less, the adhesion force due to the residual charge increases and the transfer sheet reaches the separating pawl, which causes scum of the transfer sheet.
The ease with which the transfer sheet separates is affected not only by the repulsion phenomenon described above or the residual charge, but also by the effect of the transfer bias. In other words, transfer bias is essential to effect a good charging of the belt. However, the effect of the bias varies according to the environmental conditions. That is, the charging of the belt differs according whether bias is impressed under conditions of low temperature low humidity or high temperature high humidity. Thus, impressing uniform transfer bias does not guarantee separation of the transfer sheet even if uniform neutralization is carried out by means of pre-transfer exposure.
The phenomenon of variation of charging properties according to environmental conditions is explained by taking an instance in which a moisture-absorbing paper is used as the transfer sheet.
When a double-side image formation mode is selected, a different moisture content setting is applicable when forming image on the second surface than when forming image on the first surface. The initial moisture content is set less for the second surface due to the presence of the fixer used on the first surface. Thus, the ability of the transfer sheet to separate from the photosensitive drum may vary according to the charging properties of the transfer sheet, which varies according to whether both sides are used for image formation or only a single side of different transfer sheets having varying moisture content is used or whether a composite image is formed on a single side.
The ability of the transfer sheet to separate tends to deteriorate if the transfer sheet having moisture content, etc., has inferior inherent charging properties, in particular, resistance. Further, the edge of the transfer sheet tends to curl up, making it prone to get in the way of the photosensitive drum. That is, the transfer sheet does not exactly sit flush on the belt due to the curling up of the edge.
Apart from uneven neutralization of the surface of the photosensitive drum, another factor that causes deterioration of the ability of the transfer sheet to separate from the photosensitive drum is the tendency of the separating pawl to deteriorate with time.
The separating blade tends to deform and abrade with time. Due to this, the transfer sheet that is electrostatically adhering to the photosensitive drum tends to jam at the site of the separating blade and is carried to the cleaning apparatus still stuck to the separating blade.